Cold spraying particles onto a substrate surface to protect the substrate has been gaining increased acceptance as a viable method of coating a substrate. To obtain high-performance coatings the cold spraying is conducted at a high pressure with the assistance of a high-pressure gas, such as, for example, helium, nitrogen, and air having a coating material infused therein, which includes, for example, powder metals, refractory metals, alloys and composite materials. Powder particles having a size range of between about 20 to 50 micrometers are introduced at a high pressure into a supersonic gas stream generated by a spray gun and emitted from a nozzle. One such nozzle is disclosed in U.S. Pat. No. 8,132,740, the contents of which are incorporated herein by reference. The powder particles are accelerated to a supersonic velocity and directed to impact the substrate onto which the coating is to be formed.
Kinetic energy generated from impact of the particles on the substrate causes the particles to deform to a slightly flat configuration and diffuse into the substrate. The deformation promotes adhesion to the substrate, interlocking between adjacent particles and the substrate, and metallurgical bonding with the substrate resulting in a protective coating on the substrate. Because the particles are cold sprayed at near ambient temperatures, oxidation while airborne and forming the coating is prevented or significantly reduced.
However, because the distribution of the particles is not uniform and random, the structures of the coating and performance properties are not believed to be optimized. An effort to enhance the performance properties of the coating applied through conventional cold spraying includes a step of heat treatment or annealing of a cold spray coating in a furnace or by way of laser heating. However, heat treating or annealing the cold spray coatings is known to decrease the mechanical properties while resulting in more complexity and cost associated with cold spraying a substrate. Further, a laser heating process located adjacent the cold spraying operation is not viable due to airborne particles proximate the area of deposition and the inability to control necessary laser strength and other parameters to provide the desired annealing of the cold spray coating.
Coatings applied by high pressure cold spraying processes are believed provide desirable durability properties. However, it is difficult to perform high pressure cold spraying in a conventional industrial environment without enclosing the high pressure cold spray system within a spray booth, cabinet, and helium and/or nitrogen shrouds to achieve the high particle velocity and prevent oxidation of the particles, which increases manufacturing complexity and cost. High pressure cold spray processes generate particle velocity in the range of 550 m/s to 900 m/s requiring environmental containment.
One solution to some of these drawbacks of high pressure cold spraying technology is to reduce pressure of the cold spray nozzle to a speed of about 300 m/s to 500 m/s or a low pressure cold spray. However, low pressure cold spraying coatings provide an undesirable structure that does not perform well when compared with high pressure cold spray coatings. This is believed to be a result of insufficient particle velocity not providing desired particle deformation and resulting in weaker particle bonds and undesirable porosity of the resulting coating.
Therefore, it would be desirable to provide a low pressure cold spray process that provides desired particle deformation, particle bonding, and coating porosity.